The present invention is generally in the area of synthetic materials for bone repair and replacement, and is particularly a poly(organophosphazene) three dimensional matrix.
Successful design of an implant to replace skeletal tissue requires knowledge of the structure and mechanical properties of bone and an understanding of the means by which grafts become incorporated into the body. This information can then be used to define desirable characteristics of the implant to ensure that the graft functions in a manner comparable to organic tissue.
A graft may be necessary when bone fails and does not repair itself in the normal amount of time or when bone loss occurs through fracture or tumor. Bone grafts must serve a dual function: to provide mechanical stability and to be a source of osteogenesis. Since skeletal injuries are repaired by the regeneration of bone rather than by the formation of scar tissue, grafting is a viable means of promoting healing of osseous defects, as reviewed by Friedlaender, G. E., "Current Concepts Review: Bone Grafts," Journal of Bone and Joint Surgery, 69A(5), 786-790 (1987). Osteoinduction and osteoconduction are two mechanisms by which a graft may stimulate the growth of new bone. In the former case, inductive signals of little-understood nature lead to the phenotypic conversion of connective tissue cells to bone cells. In the latter, the implant provides a scaffold for bony ingrowth.
The bone remodeling cycle is a continuous event involving the resorption of pre-existing bone by osteoclasts and the formation of new bone by the work of osteoblasts. Normally, these two phases are synchronous and bone mass remains constant. However, the processes become uncoupled when bone defects heal and grafts are incorporated. Osteoclasts resorb the graft, a process which may take months. More porous grafts revascularize more quickly and graft resorption is more complete. After the graft has been resorbed, bone formation begins. Bone mass and mechanical strength return to near normal.
Present methods for the repair of bony defects include grafts of organic and synthetic construction. Three types of organic grafts are commonly used: autografts, allografts, and xenografts. An autograft is tissue transplanted from one site to another in the patient. The benefits of using the patient's tissue are that the graft will not evoke a strong immune response and that the material is vascularized, which allows for speedy incorporation. However, using an autograft requires a second surgery, which increases the risk of infection and introduces additional weakness at the harvest site. Further, bone available for grafting may be removed from a limited number of sites, for example, the fibula, ribs and iliac crest. An allograft is tissue taken from a different organism of the same species, and a xenograft from an organism of a different species. The latter types of tissue are readily available in larger quantities than autografts, but genetic differences between the donor and recipient may lead to rejection of the graft.
Synthetic implants may obviate many of the problems associated with organic grafts. Further, synthetics can be produced in a variety of stock shapes and sizes, enabling the surgeon to select implants as his needs dictate, as described by Coombes, A. D. A. and J. D. Heckman, "Gel Casting of Resorbable Polymers: Processing and Applications," Biomaterials, 13(4), 217-224 (1992). Metals, calcium phosphate ceramics and polymers have all been used in grafting applications.
Calcium phosphate ceramics are used as implants in the repair of bone defects because these materials are non-toxic, non-immunogenic, and are composed of calcium and phosphate ions, the main constituents of bone (Jarcho, 1981; Frame, J. W., "Hydroxyapatite as a biomaterial for alveolar ridge augmentation," Int. J. Oral Maxillofacial Surgery, 16, 642-55 (1987); Parsons, et al. "Osteoconductive Composite Grouts for Orthopedic Use," Annals N.Y. Academy of Sciences, 523, 190-207 (1988)). Both tricalcium phosphate (TCP) [Ca.sub.3 (PO.sub.4)2] and hydroxyapatite (HA) [Ca.sub.10 (PO.sub.4).sub.6 (OH.sub.2 ] have been widely used. However, the mechanical properties of calcium phosphate ceramics make them ill-suited to serve as a structural element. Ceramics are brittle and have low resistance to impact loading.
Over the last decade there has been a tremendous increase in applications for polymeric materials. They have been used widely for surgical implants, and artificial organs, as reviewed by D. K. Gilding and A. M. Reed, "Biodegradable polymers for use in surgery: Polyglycolic acid/polylactic acid homo-and copolymers," Polymer 20, 1459-1464 (1979); J. O. Hollinger and G. C. Battisone, "Biodegradable bone repair materials: Synthetic polymers and ceramics," Clin. Orthop., 207, 290-305 (1986); L. Cima, et al. "Tissue engineering by cell transplantation using degradable polymer substrates," J. Biomech., Eng., 113, 143-151 (1991); C. A. Vacanti, et al. "Synthetic polymers seeded with chrondrocytes provide a template for new cartilage formation," Plast. Reconstr. Surg., 88, 753-759 (1991); J. O. Hollinger, "Preliminary report on the osteogenic potential of a biodegradable copolymer of polylactide (PLA) and polyglycolide (PGA)." J. Biomed. Mater. Res. 17, 71-82 (1983); and P. Potin and R. De Jaeger, "Polyphosphazenes: Synthesis, structures, properties, applications," Eur. Polym. J., 27, 341-348 (1991). These materials are well suited to implantation as they can serve as a temporary scaffold to be replaced by host tissue, degrade by hydrolysis to non-toxic products, and be excreted, as described by Kulkarni, et al., J. Biomedical Materials Research, 5, 169-81 (1971); Hollinger, J. O. and G. C. Battistone, "Biodegradable Bone Repair Materials," Clinical Orthopedics and Related Research, 207, 290-305 (1986). Four polymers widely used in medical applications are poly(paradioxanone) (PDS), poly(lactic acid) (PLA), poly(glycolic acid) (PGA), and PLAGA copolymers. Copolymerization enables modulation of the degradation time of the material. By changing the ratios of crystalline to amorphous polymers during polymerization, properties of the resulting material can be altered to suit the needs of the application.
These polymers, including poly(lactide-co-glycolic) acid (PLAGA), have been used as polymer composites for bone replacement as reported by H. M. Elgendy, et al. "Osteoblast-like cell (MC3T3-E1) proliferation on bioerodible polymers: An approach towards the development of a bone-bioerodible polymer composite material," Biomaterials, 14, 263-269 (1993). Substituted polyphosphazenes have been shown to support osteogenic cell growth, as reported by C. T. Laurencin, et al. "Use of polyphosphazenes for skeletal tissue regeneration," J. Biom. Mater. Res., 27 (1993). Coombes and Heckman 1992 and Hollinger 1983 have attempted to create poly(lactide-co-glycolide) [(C.sub.3 H.sub.4 O.sub.2).sub.x (C.sub.2 H.sub.2 O.sub.2).sub.y ] implants as bone substitute. Although initial results by Hollinger suggested that polymer may aid in osteoinduction in the early bone repair process, by 42 days, the rate of repair was equivalent in controls and experimental defect sites. As reported by Coombes and Heckman, after eight weeks degradation in phosphate buffered saline (PBS), the strength of the material had deteriorated significantly. Moreover, the microporous structure (pores 205 .mu.m in diameter) has been shown to be too small to permit the ingrowth of cells, as reported by Friedlander, G. E. and V. M. Goldberg, Bone and Cartilage Allografts, Park Ridge: American Academy of Orthopedic Surgeons, 1991; Jarcho, M. "Calcium Phosphate Ceramics as Hard Tissue Prosthetics," Clinical Orthopedics and Related Research, 157, 259-78 (1981). Accordingly, from a mechanical as well as a biological standpoint, this matrix is not ideal for use as a substitute bone graft material.
Poly(organophosphazenes) are high molecular weight polymers containing a backbone of alternating phosphorus and nitrogen atoms. There are a wide variety of polyphosphazenes, each derived from the same precursor polymer, poly(dichlorophosphazene). The chlorine-substituted species can be modified by replacement of the chlorine atoms by different organic nucleophiles such as o-methylphenoxide along with amino acids. The physical and chemical properties of the polymer can be altered by adding various ratios of hydrolytic sensitive side chains such as ethyl glycinate, as described by C. W. R. Wade, et al. "Biocompatibility of eight poly(organophosphazenes)," in Organomet. Polym., C. E. Carraher, J. E. Sheats and C. U. Pitman, Jr., Eds., Academic Press, New York, 1978, pp. 283-288; and H. R. Allcock and T. J. Fuller, "Synthesis and Hydrolysis of Hexakis(imidazolyl)cyclotriphosphazene," J. Am. Chem. Soc., 103, 2250-2256 (1981). This will affect the degradation of the polymer as an implantable and biodegradable material as well as vary the support of osteogenic cells for bone and tissue implants, as shown by Laruencin, et al. (1993). However, even with knowledge of degradation properties of the polymer, as in the case of PLGA, it is still necessary to develope a three dimensional matrix system in order to maximize growth, increase cell attachment and promote permanent fixation by ingrowth of living tissue which has desireable properties in vivo.
It is therefore an object of the present invention to provide a three dimensional matrix for regeneration of skeletal tissues which is biocompatible and biodegradable, and methods for preparation thereof.
It is a further object of the present invention to provide methods to manipulate cell growth on synthetic biocompatible, biodegradable polymeric matrices.